Infection of humans with the zoonotic bacterium Bartonella henselae (Bh) can result in a range of clinical symptoms and disease including lymphadenopathy associated with localized Bh infection observed in cat-scratch disease, to endocarditis and bacteremia resulting from systemic disease. In some patients, systemic infection with Bh includes bacillary angiomatosis which is characterized by red/purple nodules on the skin or internal organs due to proliferation of the small blood vessels. This unusual angiogenic host response to infection is unique among bacteria to certain Bartonella species. In vitro studies have defined important virulence factors in Bh that play a role in establishing infection and/or eliciting a host response. The first is the VirB/VirD4 type IV secretion system that is responsible for delivering the bartonella effector proteins (Beps A-G) that act on endothelial cells in varying capacities. The second major virulence factor is a VirB-independent second type IV secretion system Trw, which encodes multiple proteins including small adhesins TrwL1-TrwL-8 which have been shown to play an important role in host cell adherence. Lastly, the high molecular weight trimeric auto transporter adhesin BadA has been studied extensively in vitro and is critical for auto agglutination, adhesion to host cells, binding to extracellular mtrix proteins, inhibition of phagocytosis, and induction of angiogenesis. While in vitro studies have proven invaluable for the study of Bh pathogenesis, no Bh virulence factors have been confirmed in vivo and remarkably little is known about how the genes encoding these virulence factors are regulated. To address this problem, we have developed a novel in vivo model of Bh infection using transgenic zebra fish (ZF) embryos that permits us to both monitor infection and host response including angiogenesis. We hypothesize that our ZF embryo model will enable us to: more rigorously test the role of virulence factors that have been defined in vitro; identify ne virulence factors and regulators of virulence in vivo ; and assess the role of these gene products in the host response to Bh infection. To test these hypotheses we propose the following specific aims: 1) Define the role of virulence factors and regulators of virulence in Bh infection using the transgenic ZF embryo model, and 2) Determine the role of Bh virulence factors and regulatory proteins in eliciting a host response to infection in experimentally infected transgenic ZF embryos. An in vivo model is urgently needed to better understand the unique pathogenesis and host response to this bacterium in order to develop novel treatment strategies.